Correlation between sub-gap states and optoelectronic properties of amorphous
zinc tin oxide
Esteban Rucavado
1,*
, Federica Landucci
1
, Quentin Jeangros
1
, Jakub Holovský
2
, Aïcha Hessler-Wyser
1
,
Monica Morales-Masis
1
, Christophe Ballif
1
1
Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics
and Thin-Film Electronics Laboratory, Rue de la Maladière 71b, 2002 Neuchâtel, Switzerland
2
Czech Technical University (CTU), Faculty of Electrical Engineering, Technická 2, 166 27 Prague,
Czech Republic
Historically, tin-doped indium oxide (ITO) has dominated the market of transparent conductive materials
(TCMs) for optoelectronic applications, due to its good electrical and optical properties. But indium is a
scarce element, hence urging the development of suitable indium-free TCM replacements with properties
as good as indium-based oxides. Amorphous zinc tin oxide (a-ZTO) is a wide band gap alloy composed
exclusively of Earth abundant elements and a previous study demonstrated that a-ZTO, with a
stoichiometry of Zn
0.06
Sn
0.28
O
0.66
, can replace ITO in large-area white OLEDs [1]. The high performance
of a-ZTO is attributed to its low absorptance (< 5% from the visible VIS to the near infrared NIR), dense
(void free) microstructure, and sufficient conductivity (200 Ω
-1
cm
-1
). Yet its Hall mobility of ~20 cm
2
V
-1
s
-1
and free carrier concentration of ~7 x 10
19
cm
-3
are low compared to In-based TCMs. An in-depth study is
hence performed to assess the carrier transport limiting mechanisms in a-ZTO along with the effects of
post-deposition treatments on the optoelectronic properties of a-ZTO. We discuss how a high density of
defects creates sub-bandgap states, increasing the absorptance in the visible region of the spectra and
affecting the mobility of the films. Based on this study, solutions to passivate these defects and improving
the mobility of the films are demonstrated.
For this research, a-ZTO films 150 nm-thick were sputtered and annealed at temperatures from 150°C to
500°C in oxygen rich and oxygen poor atmospheres, i.e. in air (1 bar) and in N
2
(0.5 mbar). Upon
annealing for 30 minutes in these conditions, a-ZTO films preserve their dense amorphous microstructure
as confirmed by X-ray and electron diffraction. When annealed at 150°C, a-ZTO presents slightly
improved electrical properties while preserving its optical properties. More significantly, after annealing at
500°C in air, an increase in mobility up to 35 cm
2
V
-1
s
-1
is obtained together with an improvement of
optical properties, as the free carrier density decreases to 5 x 10
19
cm
-3
. In contrast, thermal treatments in
N
2
result in an increase of carrier concentrations to 13 x 10
19
cm
-3
and unchanged mobility. Photo-thermal
deflection spectroscopy (PDS) was performed to determine sub-band gap defects and the Urbach energy of
a-ZTO. This technique highlights a high density of states in the forbidden energy band in the as-deposited
state, that are suppressed after annealing at 500°C in air. Annealing in N
2
atmosphere at 500°C decreases
slightly the density of these sub-gap states. From a combination of temperature-dependent Hall-effect
measurements, Drude-model fitting from FTIR measurements and Brooks-Herring-Dingle modeling, we
show that the dominant scattering mechanism for electrons in a-ZTO is ionized point defects, namely
single and double-charged oxygen vacancies (
V
o
). These are the main source of free electrons [2] and form
sub-band gap defect levels in a-ZTO. Finally, we demonstrate that these sub-band gap defect levels formed
by the ionized impurities in the films are passivated in air at high temperatures. Preliminary results suggest
that is also possible to passivate these defects at low temperatures (< 200°C) by co-depositing a-ZTO with
high-dielectric metal oxides or by oxygen-rich post-deposition treatments. The stability of a-ZTO at high
temperatures, together with the improvement in optoelectronic properties, makes this material interesting
for application in PV or other large-area electronic devices which require high processing temperature. In
addition, the low deposition temperature coupled to a possible defect passivation at low temperature
expands its application to temperature-sensitive processes.
[1]
M. Morales-Masis
et al.
,
Adv. Funct. Mater, 2016, 26, p.384-392.
[2]
Q. Zhu
et al.
J. Appl. Phys.
,
2014, 115
,
p. 033512-1-9.
*
Electronic mail :
O 8
-35-
